1. Field of the Invention
This invention relates to an injection compression molding method for compact discs or the like, and in particular, to an injection compression molding method which copes with the problem of residual stresses, which problem occurs as the production cycle for compact discs or the like is shortened.
2. Description of the Related Art
In the production of compact discs, for example, it has been the common practice to provide a sufficient length of cooling time, approximately 6 to 8 seconds, when cooling each optical disc base with a view to mitigating the various stresses involved, i.e., filling pressure, dwell pressure, clamping pressure, compression pressure, etc. To reduce the production costs, however, it is necessary to shorten the above-mentioned cooling time. In view of this, various attempts have been made to improve the cooling efficiency. Examples of such attempts include: bringing the cooling water channel nearer to the disc surface; widening the channel so as to increase the water flow; and selecting a mold material which has a higher heat conductivity. These arrangements, on the other hand, have not been without their problems: the disc surface layer is apt to become hardened at an early stage; a difference in stress is apt to be generated between the inside and the exterior surface portion of the disc; and the optical properties of the disc, such as birefringence, are deteriorated.